Paroxysmal nocturnal hemoglobinuria (PNH) is a chronic hemolytic disorder characterized by the loss of membrane proteins that protect erythrocytes from complement mediated lysis. It is associated with a high risk of major thrombotic events, most commonly thrombosis of large intraabdominal veins. Most patients who die of their disease die of thrombosis. While the cause of this thrombophilia is unknown, we have recently demonstrated normal levels of platelet activation in patients with PNH. We have written a protocol that is currently under consideration by the NHLBI IRB (minor changes have been requested) for treatment of patients with PNH with aspirin. Patients without a history of thrombosis or bleeding, and with platelet counts over 50,000/mm3, will be screened for platelet activation on at least two occasions by flow cytometry. Patients with evidence of activation will be placed on low-dose aspirin for two weeks, and their degree of platelet activation followed. They will then be reassessed after a period off aspirin.